The present invention relates to wavelength division multiplexed (WDM) optical communications systems, and more particularly to optical attenuation across a plurality of optical channels and/or compensation for chromatic dispersion and chromatic dispersion slope in WDM optical communication systems.
Wavelength division multiplexing (WDM) is a method by which a plurality of signal-carrying lights, each such light comprising a specific, restricted wavelength range, are carried along an optical fiber communications system. In this specification, these individual information-carrying lights are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannels.xe2x80x9d The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a xe2x80x9ccomposite optical signal.xe2x80x9d
The term xe2x80x9cwavelengthxe2x80x9d is used synonymously with the terms xe2x80x9csignalxe2x80x9d or xe2x80x9cchannel.xe2x80x9d Although each information-carrying channel actually comprises light of a certain restricted range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength and a plurality of such channels are referred to as xe2x80x9cwavelengthsxe2x80x9d. Used in this sense, the term xe2x80x9cwavelengthxe2x80x9d may be understood to refer to xe2x80x9cthe channel nominally comprised of light of a range of physical wavelengths centered at a particular nominal wavelength.xe2x80x9d
Fiber optic networks are becoming increasingly popular for data transmission because of their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed.
Because optical signals lose intensity upon transmission over long distances through optical fiber, optical amplifiers are commonly employed within optical communications systems to boost the signal intensity. The most common example of an optical amplifier is the Erbium Doped Fiber Amplifier (EDFA), for which an exemplary gain spectrum 10 is illustrated in FIG. 1. FIG. 1 shows that, although an EDFA can increase the signal intensity significantly, the gain that it produces is not uniform over the entire optical transmission region. This non-uniform gain does not pose a problem for single-channel (-wavelength) optical communications systems. However, for multi-channel (wavelength division multiplexed) systems, the non-uniform gain leads to a well-known problem of non-uniform amplification of the various channels. For instance, if the wavelengths xcex107 and xcex108 are used to represent two such channels amplified by a single EDFA, then it can be seen from FIG. 1 that the longer wavelength channel xcex107 receives a lesser amount of amplification or gain than does the shorter wavelength channel xcex108. After being output from the EDFA, the two channels xcex107 and xcex108 will exhibit non-equivalent intensities, which is an unacceptable result. Additional wavelength division multiplexed channels between xcex107 and xcex108 will exhibit a non-constant intensity distribution approximately described by the dashed line 11 in FIG. 1.
An even greater problem with the use of EDFA""s is the fact that the exact form of the gain spectrum 100 is not static but can vary depending upon the amount of optical power that is input to an EDFA. This is most evident as a change in the gain tilt, which is the slope of the line 11 representing an average variation of the gain between the wavelengths xcex107 and xcex108. With changing gain tilt, the difference in amplification between channels is not constant.
A second common and well-known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect wherein the individual wavelengths comprising an optical channel travel through an optic fiber at different speeds. This is a particular problem that becomes more acute for data transmission speeds higher than 2.5 gigabytes per second. The resulting pulses of the signal will be stretched, will possibly overlap, and will cause increased difficulty for optical receivers to distinguish where one pulse begins and another ends. This effect seriously compromises the integrity of the signal. Therefore, for a fiber optic communication system to provide a high transmission capacity, the system must compensate for chromatic dispersion. The exact value of the chromatic dispersion produced in a channel of a wavelength-division multiplexed fiber optic communications system depends upon several factors, including the type of fiber and the wavelength of the channel. Chromatic dispersion slope is the variation of the chromatic dispersion amongst the various channels comprising a WDM composite optical signal.
Conventional apparatuses that can be used as dispersion compensating components include dispersion compensation fiber, chirped fiber Bragg gratings coupled to optical circulators, and conventional diffraction gratings disposed as sequential pairs. Unfortunately, these conventional apparatuses do not compensate for unequal channel intensities produced by EDFA gain tilt.
Accordingly, there is a need for an improved gain slope equalizer. The gain slope equalizer should provide variable optical attenuation of a composite optical signal so as to equalize the intensities of a plurality of WDM channels so as to compensate for gain slope. It should be able to be used in an apparatus which provides non-uniform chromatic dispersion so as to compensate for fiber-induced chromatic dispersion and dispersion slope. The present invention addresses such a need.
The present invention provides an improved gain slope equalizer which provides variable optical attenuation. The gain slope equalizer includes a transmission diffraction grating with a first side and a second side; a first lens optically coupled to the second side of the transmission diffraction grating; and at least one reflective surface optically coupled to the first lens at a side opposite to the transmission diffraction grating. The gain slope equalizer in accordance with the present invention can also be used with a Virtually Imaged Phased Array (VIPA) to provide a chromatic dispersion slope and chromatic dispersion compensation as well as variable optical attenuation. The present invention provides the heretofore unavailable capability of simultaneous tunable gain slope equalization and chromatic dispersion compensation utilizing a single apparatus.